An electronic control system, which is mounted in a vehicle and controls various equipment such as an engine, performs diagnosis operation (determination of normality or abnormality) about various check items based on data (information) from sensors mounted in the vehicle. If any abnormality is determined, abnormality data (diagnostic trouble code DTC or fault code) indicating a diagnosis result of abnormality is stored in a standby RAM (SRAM). This standby RAM is a backup RAM, which continues to store its storage contents if electric power is continuously supplied thereto by a battery mounted on the vehicle, even if electric power supply to an electronic control unit (ECU) is interrupted.
If the electric power supply by the battery of the vehicle is interrupted due to battery cable disconnection or battery rundown, the standby RAM will lose its storage contents and cannot continue to store the DTC permanently.
The California Air Resources Board (CARB) provides in its legislation OBD2 the following regulation (a).
(a) The DTC shall be stored as a permanent fault code (PDTC), which is not erased even when the electric power supply is interrupted by the battery disconnection or the battery rundown. To meet this requirement, the ECU is configured to store the DTC as the PDTC in a nonvolatile memory, which is a rewritable memory such as an EEPROM.
The legislation OBD2 also provides the following regulations (b) to (f).
(b) The PDTC shall not be erased by a command from an external tool, which is connectable to and communicable with the ECU.
(c) The PDTC shall be stored up to a minimum of four.
(d) The PDTC may be erased by the ECU after determination of normality in each of three driving cycles. Each driving cycle (DCY) is an interval from starting an engine to restarting the engine after stopping once.
(e) The PDTC may be erased by the ECU after determination of normality in one DCY after clearing data stored in a volatile memory by disconnecting the battery.
(f) If any failure is detected, the PDTC shall be stored before the DCY, in which the failure is detected, ends.
In some instances, the ECU is operated before assembling the ECU to a vehicle in manufacturing the vehicle is completed, that is, before all peripheral devices such as sensors, electric actuators and the like are completely assembled in the vehicle as designed.
If the ECU performs its diagnosis operation under such a condition, it is likely that the ECU will likely to detect abnormality and store DTC indicating such detected abnormality in a rewritable nonvolatile memory. This DTC produced before the completion of assembling of the ECU to the vehicle is abnormality data produced in the course of assembling operation and before completion of manufacture of the vehicle.
Some ECUs are switchable between a function check mode and a normal mode to operate in either the function check mode or the normal mode. The function check mode is provided so that the ECU is allowed to perform its function check operation different from the normal operation.
In case of such an ECU, the ECU is first operated under the function check mode after being assembled to the vehicle. If no abnormality is confirmed in the operation of the function check mode, it is determined that the assembling of the ECU to the vehicle has been completed. After this determination, the ECU is switched from the function check mode to the normal mode to operate in the normal mode. In this instance, the DTC indicating detection of abnormality in the function check mode is unnecessarily stored in the rewritable nonvolatile memory, even though the DTC is produced before the completion of assembling of the ECU to the vehicle.
If the ECU is configured to detect abnormality more sensitively in the function check mode than in the normal mode, more DTCs produced in the function check mode will be stored in the rewritable nonvolatile memory.
It is often tried to check whether the ECU can actually detect abnormality in the function check mode by intentionally generating abnormal condition. In this case, too, the DTC produced in the function check mode before the completion of assembling to the vehicle will be stored in the rewritable nonvolatile memory. Such a DTC produced before the completion of assembling of the ECU to the vehicle is not necessitated in the normal vehicle maintenance or check work in the market.
Such a DTC produced before the completion of assembling is read out as the PDTC, when the PDTC in the rewritable nonvolatile memory is read out by a failure diagnosing apparatus after the vehicle has been actually sold and used in the market. This PDTC will erroneously indicate abnormality in the vehicle maintenance shop.
It is therefore proposed in JP 2006-291730 A to allow an ECU to store diagnosis result in a memory only after a predetermined condition has been satisfied, for example, only after a user has actually started using a vehicle. According to this proposal, however, it becomes absolutely impossible to read out abnormality data indicating abnormality detected in the course of assembling the ECU to the vehicle and to identify the abnormality later.